Health workers trying to contain a large Ebola outbreak in Congo are facing more than the threat of a deadly, fast-moving virus.

In the past week, armed assailants in Congo have attacked two Ebola clinics run by Doctors Without Borders, threatening an urgent response to the medical crisis and prompting the humanitarian group to suspend its operations in the epicenter of the outbreak. There have been at least 879 confirmed cases of Ebola in Congo in the past seven months, and 553 people have died of the virus.

On Sunday evening, a group of people began to throw rocks at an Ebola treatment clinic in the district of Katwa in Congo’s North Kivu province, then set the facility on fire. No patients or staff were harmed, but one patient’s caretaker died. Then, on Thursday, a group attacked another nearby Ebola facility in Butembo, setting the building and some of its vehicles on fire. Doctors Without Borders, also known by its French acronym, MSF, said there were 57 patients in its center that was attacked Thursday, and 15 of them had confirmed cases of Ebola.

MSF evacuated its staff from the area, and all patients had to be moved from the facility and placed in a transit center. The Ministry of Health announced that four patients, who were highly contagious, are now missing.

“For the family of the patients who were in the centers when they were attacked, for our staff who were there, it was very traumatizing,” said Trish Newport, who served as emergency coordinator for MSF in Congo from October to February.

“Ebola is already scary,” she said. “It’s scary for the population, it’s scary for the patients, and to have this on top of that is terrifying.”

Part of what makes it even scarier is that health workers don’t always know who is targeting them. “We don’t know their motivation,” Newport said of the groups that attacked their clinics in the past week, noting that vaccination teams and other health workers have had rocks thrown at them before.

As in other Ebola responses, health workers in Congo are working to try to earn the trust of local communities where the virus is spreading. Parts of Congo’s North Kivu province are volatile, and violent armed groups have slowed the Ebola response there. And after years of conflict, some communities in the area are fearful and suspicious of outside intervention.

Newport said that Ebola often mimics the symptoms of other diseases people may be more familiar with, like malaria or typhoid. Finding ways to explain to the community why Ebola must be treated so differently — and with such intensity — can be a challenge.

Last week, 85 percent of all new confirmed cases in Congo were diagnosed in Butembo and Katwa, the two areas where Doctors Without Borders’ Ebola services have now been suspended, she said. And part of what is most concerning to health-care workers there is that in about half those cases, presence of Ebola was not confirmed until after the patient had died, signaling that they had probably been sick for a long time. That increases the likelihood that they transmitted the virus to others.

For a new Ebola vaccine to be effective, health workers have to identify a “ring” of people an infected patient may have come into contact with while they were sick. Then they track down each of those people, monitor their temperatures and provide them with a vaccine. The idea is that if they can adequately protect each person who came into contact with an Ebola patient, they can quickly slow the spread of the virus.

When a patient isn’t diagnosed until after their death, it makes it much harder to assess whom they may have been in contact with in their final weeks and raises the possibility they exposed a larger group to infection over a longer period.

These challenges, paired with security risks in the region, make a coordinated response complicated on multiple levels. But the consequences for not addressing the outbreak head-on could be enormous.

When Ebola broke out in West Africa between 2014 and 2016, it spread rapidly in some places, killing more than 11,000 people.